The present invention relates to a method of lithography using electromagnetic wave and resist sensitive to that wave for micro-fine pattern of 1 .mu.m or less on a semiconductor substrate, and more particularly to X-ray lithography.
The exposure technique by electromagnetic wave having shorter wave length than visible light or ultraviolet light, such as X-ray or electron beam, is expected as a reprint technique that has great potential in the future because of reliable high resolution for a submicron width pattern and, at present, vigorous research and development are being carried out at various places. The technique is explained, for example, in a publication issued in 1972, "Electronics Letters" Vol. 8, No. 4, p.p. 102.about.104. The X-ray or electron beam exposure technique in the prior art, however, requires a long time to reprint a pattern in an X-ray or electron beam resist layer provided on a substrate such as a semiconductor wafer and, therefore, the throughput in the technique is low. Factors determining the throughput in the X-ray or electron beam exposure are the intensity of an X-ray or electron beam source and the sensitivity of the resist. In the case of an X-ray source, the most popularly used source is of a type which can emit X-rays from a target by impacting an electron beam on to the target. However, the conversion efficiency of the electron beam output to X-rays is as extremely low (i.e. .about.10.sup.-4) and it is a diverging radiation source.
Therefore, X-rays obtained by the source are weak. On the other hand, a plasma X-ray source which is expected as a high-luminescence X-ray source is a pulsed X-ray source, and a conversion efficiency for one pulse is .gtoreq.10.sup.-2, but in a practical use, many problems must be resolved such as various damage that results from generation of pulses. In addition, with regard to a synchrotron orbital radiation source for which a trend of research and development has developed recently, it is too early to determined its practicality. Accordingly, so long as an X-ray source concerns, at present, it can be said that the source excited by the electron beam is most suitable for commercial X-ray exposure apparatus. In this respect, that is, in the case of employing the source excited by the electron beam, particularly a high sensitivity of X-ray resist is necessary. In order to obtain a sufficient throughput, development of X-ray resist having a high sensitivity of 10 mJ/cm.sup.2 or less is necessary. However, X-ray resist . having such a high sensitivity and utility is readily not available at present. It is believed that a considerable period of development in the future will be necessary until X-ray resist having a sensitivity of 10 mJ/cm.sup.2 or less, which is applicable to fabrication of practical devices, is available. In commercial exposure apparatuses, one of the most important factors is the throughput and, as described above, in the X-ray exposure technique also, enhancement of the throughput is an important problem. The situation is similar for the case where electron beam exposure techniques are used.